The use of this particular tilted mortise feature as explained in the email would work for roughly half of what would be necessary for a very limited chair design. A relatively simple design where the two side assemblies are mirror images, as they are here:
(more of a simplified and squared stool design, than a traditional splayed and angled seat design).
Not being a fan of angled tenons in chair making, and if the goal is to design/fabricate using a traditional chair design (with more involved compound angled joinery as shown below) the tilting "feature" of the mortiser arguably won't take you that far:
In that respect, the explanation in the email, as justification for the limited ability to tilt, is arguably misleading.
Particularly to a novice purchaser, purchasing in part based on the advertised feature, but not fully aware of what it takes to effect M&T chair joinery, fixed or floating tenon.
Not everyone would stop to think it out, as you have.